The present invention relates generally to machining metal, and, more specifically, to polishing thereof.
A gas turbine engine includes various compressor and turbine stator vanes and rotor blades. The vanes and blades have airfoil profiles specifically configured for use in compressing air or expanding hot combustion gases.
Accordingly, the airfoil profiles have three dimensional contours which vary from root to tip between leading and trailing edges over the opposite pressure and suction sides thereof. The airfoil has camber or arcuate curvature between its leading and trailing edges, with the pressure side typically being generally concave and the suction side typically being generally convex.
The airfoils may be made by various manufacturing processes. For example, some compressor airfoils are machined to size from metal blanks or forgings. Computer numerically controlled machines are commercially available for precisely controlling the position of a ball endmill tool over the complex three dimensional contour of the airfoil. Machining typically occurs by tracing the tool around the pressure and suction sides of the airfoil at a specific radial or span location, and then indexing the tool radially along the span for machining each radial section in turn from root to tip.
Since the ball endmill tool forms a small groove as it removes material around the circumference of the airfoil, adjoining grooves along the span of the airfoil have sharp ridges extending therebetween. After the machining process, these ridges must be removed to obtain an aerodynamically smooth surface finish without irregularity.
The ridges are typically removed by additional hand and machine polishing processes which increase the cost of manufacture thereof. Manual hand polishing uses grinders and buffers for removing a majority of the ridges. However, manual polishing is insufficient for meeting the required smooth finish requirements for the airfoil.
Mechanical polishing follows the hand polishing process in which a small batch of hand polished airfoils are tumbled in an abrasive bed in a vibratory finishing machine for achieving the desired smooth surface finish.
However, since a rotor blade airfoil typically includes an enlarged root end with an integral platform, the platform acts as an anchor in the polishing bed which results in directional polishing along the span or longitudinal axis of the airfoil.
These multiple steps in polishing the machined airfoils increase the time of manufacture and corresponding cost thereof for achieving the desired smooth airfoil surface devoid of ridges and grooves.
Accordingly, it is desired to provide a new process for polishing workpieces, such as machined airfoils, with improved effectiveness and decreased cost.
A shield is placed next to a workpiece to form a passage therebetween. The workpiece and shield are rotated, and a stream of abrasive shot is blasted toward the spinning workpiece and shield to periodically enter the passage to selectively abrade the workpiece.